Control circuit for a multi-phase motor

ABSTRACT

A control circuit for a multi-phase motor (57) comprises a plurality of inverter bridges, a plurality of outputs, and at least one isolation switch (62, 63). Each inverter bridge is arranged to provide an output voltage for a phase of the motor (57). Each output is arranged to be coupled to one phase of the motor (57) to provide the output voltage to that phase of the motor (57). Each isolation switch (62, 63) is coupled between one of the inverter bridges and one of the outputs, so as to selectively isolate the output from the inverter bridge. Each isolation switch (62, 63) comprises a Gallium Nitride (GaN) transistor.

RELATED APPLICATION

This application corresponds to PCT/GB2018/053409, filed Nov. 26, 2018,which corresponds to British Application No. 1719771.6, filed Nov. 28,2017, the subject matter of which are incorporated herein by referencein their entirety.

This invention relates to a control circuit for a multi-phase motor, amotor apparatus using such a control circuit, an electric power steeringapparatus using such a motor apparatus, a vehicle using such a steeringapparatus and an associated method.

Power MOSFETs (metal oxide semiconductor field effect transistors) arecommonly used for the implementation of inverter bridges for drivingpermanent magnet synchronous motors (PMSMs) and for phase isolationfunctions in automotive applications and otherwise.

The shortcoming of designs using MOSFET for phase isolation is due tothe intrinsic body diode present in a MOSFET. This allows for theconduction of current at a low forward voltage drop (V_(SD), the forwardvoltage drop of the body diode when carrying rated current, typicallybeing less than 1V) in one direction when the MOSFET is off. This canallow damping currents which can result from voltages generated by aturning motor and under bridge MOSFET short circuit fault conditions.These damping currents are undesirable and, for example in an electricpower assisted steering (EPAS) system, can lead to a reduction insteering assistance to a user.

Such unwanted currents, can only be fully controlled by using 3 or 4MOSFET phase isolation devices as shown in FIGS. 1 and 2 of theaccompanying drawings respectively. In each case three pairs 1 and 2, 3and 4, and 5 and 6 of MOSFETs selectively connect a phase of athree-phase EPAS motor 7 to either a DC bus 8 or ground 9. For the sakeof argument, MOSFET 1 in each case is shown as having suffered a shortcircuit failure.

Arrows 10 show a potential uncontrolled current flow in the case of sucha short circuit failure, which it is possible to control using MOSFETphase isolation switches. In the example of FIG. 1, a MOSFET phaseisolation switch 11, 12, 13 in each phase is required to be able to stopthe unwanted currents, bearing in mind each MOSFET can only stop currentflowing in one direction.

In the example of FIG. 2, two MOSFETs in series 14, 15, 16, 17 areprovided for each of two phases of the motor 7. This provides the samelevel of control as for FIG. 1.

Additionally if it is the intention to control current in the two phasesnot connected to the short circuited inverter MOSFET 2, it is necessaryto block current in and out of each of the faulty inverter limbsindividually. Such a solution would require a 6 phase isolation MOSFET(18, 19, 20, 21, 22, 23) implementation as shown in FIG. 3.

As phase isolation MOSFETs are power devices that are of significantcost as well as introducing additional inserted losses into the motorcontrol circuit, it is desirable to limit the required number of devicesto achieve the phase isolation function.

In accordance with a first aspect of the invention, there is provided acontrol circuit for a multi-phase motor, the control circuit comprising:

-   -   a plurality of inverter bridges, each inverter bridge being        arranged to provide an output voltage for a phase of the motor;    -   a plurality of outputs, each output being arranged to be coupled        to one phase of the motor to provide the output voltage to that        phase of the motor; and    -   at least one isolation switch, each isolation switch being        coupled between one of the inverter bridges and one of the        outputs, so as to selectively isolate the output from the        inverter bridge;    -   in which the each isolation switch comprises a Gallium Nitride        (GaN) transistor.

As such, we have appreciated that the Gallium Nitride (GaN) transistorcan be used as an isolation switch, avoiding the problems that prior artMetal Oxide Semiconductor Field Effect Transistors (MOSFETs) involvegiven MOSFETs' inherent parasitic body diode. By using GaN transistorsas the isolation transistors, the number of components can be reduced(because there is no longer a need to employ as many components becausethere is no need to compensate for the body diode), which reducesmanufacturing cases, as well as reducing energy consumption and heatevolution.

The GaN transistor may provide the selective isolation of each isolationswitch; that is, the GaN transistor may provide a switching function toselective apply the output voltage of the inverter bridge to which itsisolation switch is coupled to the output.

Each inverter bridge which is not coupled to an isolation switch may becoupled to an output. Thus, some of the inverter bridges may be directlycoupled to the outputs without an isolation switch.

The control circuit may be provided with a controller which is arrangedto, in a normal mode of operation, close each isolation switch so thateach isolation switch does not isolate its output from its inverterbridge and allows the output voltage from its inverter bridge to beapplied to its output. As such, the controller may be arranged to applya positive voltage between a gate terminal and a source terminal of eachGaN transistor in the normal mode of operation.

The controller may also be arranged to, in a phase isolation mode ofoperation, cause at least one of the isolation switches to isolate theoutput to which it is coupled from the inverter bridge to which it iscoupled. The controller may be arranged so as to apply between the gateterminal and the source terminal, in the phase isolation mode, of eachof the isolation switches which are to be caused to isolate:

-   -   a zero voltage if the voltage across the GaN transistor is        positive; and    -   a negative voltage if the voltage across the GaN transistor is        negative.

The negative voltage applied between the gate terminal and the sourceterminal of each GaN transistor may be dependent upon the voltage acrossthe GaN transistor.

Typically, the motor will be a three phase motor, and as such, there maybe three inverter bridges and three outputs. In such a case, there maybe two or three isolation switches, dependent upon whether it is desiredcontrol the current in the phases which are not isolated. Threeisolation switches—one per phase—are required for such control.

According to a second aspect of the invention, there is provided a motorapparatus comprising a motor having a plurality of phases, and thecontrol circuit of the first aspect of the invention, in which eachphase is coupled to one of the outputs of the control circuit.

The motor may be a permanent magnet synchronous motor.

According to a third aspect of the invention, there is provided anelectric power steering apparatus, comprising a steering mechanism for avehicle and the motor apparatus of the second aspect of the invention,in which the motor is coupled to the steering apparatus so as to drivethe steering mechanism to change the steering of the vehicle. Thesteering apparatus may be an electric power assisted steering apparatus,which provides steering assistance to a driver, or may be an entirelyelectric power steering apparatus in which the motor solely drives thesteering mechanism.

According to a fourth aspect of the invention, there is provided avehicle having the electric power steering apparatus of the third aspectof the invention, in which the electric power steering apparatus isarranged to change the steering of the vehicle so as to change thedirection in which the vehicle moves.

According to a fifth aspect of the invention, there is provided a methodof controlling a control circuit for a multi-phase motor, the controlcircuit comprising:

-   -   a plurality of inverter bridges, each inverter bridge being        arranged to provide an output voltage for a phase of the motor;    -   a plurality of outputs, each output being arranged to be coupled        to one phase of the motor to provide the output voltage to that        phase of the motor; and    -   at least one isolation switch, each isolation switch being        coupled between one of the inverter bridges and one of the        outputs, so as to selectively isolate the output from the        inverter bridge;    -   in which the each isolation switch comprises a Gallium Nitride        (GaN) transistor;    -   the method comprising, in a normal mode of operation, closing        each isolation switch by causing each GaN transistor to conduct,        and in a phase isolation mode of operation, isolating at least        one output from the inverter bridge to which it is coupled by        opening at least one isolation switch by causing each GaN        transistor to cease conducting.

The method may comprise applying a positive voltage between a gateterminal and a source terminal of each GaN transistor in the normal modeof operation.

The method may comprise, in the phase isolation mode, applying betweenthe gate terminal and the source terminal of each of the isolationswitches which are to be caused to isolate:

-   -   a zero voltage if the voltage across the GaN transistor is        positive; and    -   a negative voltage if the voltage across the GaN transistor is        negative.

The negative voltage applied between the gate terminal and the sourceterminal of each GaN transistor may be dependent upon the voltage acrossthe GaN transistor.

There now follows, by way of example only, description of embodiments ofthe invention, described with reference to the accompanying drawings, inwhich:

FIG. 1 shows a circuit diagram for a motor apparatus for an electricpower assisted steering system in accordance with a first embodiment ofthe prior art;

FIG. 2 shows a circuit diagram for a motor apparatus for an electricpower assisted steering system in accordance with a second embodiment ofthe prior art;

FIG. 3 shows a circuit diagram for a motor apparatus for an electricpower assisted steering system in accordance with a third embodiment ofthe prior art;

FIG. 4 shows a circuit diagram for a motor apparatus for an electricpower assisted steering system in accordance with a first embodiment ofthe invention;

FIG. 5 shows a circuit diagram for a motor apparatus for an electricpower assisted steering system in accordance with a second embodiment ofthe invention;

FIG. 6 shows a driver circuit for use with the circuits of FIG. 4 or 5;and

FIG. 7 shows a graph showing the reverse bias characteristic of atypical Gallium Nitride transistor used in the circuits of FIG. 4 or 5,plotted as a graph of current flowing vs voltage drop across thetransistor.

A motor apparatus for an electric power assisted steering (EPAS) systemin accordance with a first embodiment of the invention is shown in FIG.4 of the accompanying drawings. Equivalent features to those discussedwith reference to FIGS. 1 to 3 above have been given correspondingreference numerals, raised by 50.

A permanent magnet synchronous motor 57 is coupled via a gearbox 70 tosteering column 71 to provide steering assistance thereto. The steeringcolumn 71 is coupled to a steering mechanism 72 which changes thedirection in which road wheels (not shown) of a vehicle 73 in which theapparatus is provided are pointing.

The motor 57 has three phases. In order to drive the motor 57 from a DCbus 58 (relative to ground 59), three inverter bridges are provided,each comprising a top switch 51, 53, 55 (selectively coupling a phase tothe DC bus 58) and a bottom switch 52, 54, 56 (selectively coupling thatphase to ground 59). These switches can be implemented as MOSFETs (metaloxide semiconductor field effect transistors).

In order to prevent unwanted currents 60 circulating in case of a top orbottom switch failing (e.g. switch 51 exhibiting a short-circuitfailure), isolation switches 62, 63 are provided in two of the phases.Rather than using MOSFETs, these switches instead employ Gallium Nitride(GaN) transistors.

GaN transistors are rapidly becoming a commercially viable alternativeto MOSFETs. The GaN transistor structure is characterised by the absenceof the parasitic bipolar junction and as a result has a differentreverse bias operation mechanism. With zero bias gate to source, thereis an absence of electrons under the gate region. As the drain voltageis decreased, a positive bias on the gate is created relative to thedrift region, injecting electrons under the gate. Once the gatethreshold is reached, there are sufficient electrons under the gate toform a conductive channel. As it takes threshold voltage to turn on theGaN transistor in the reverse direction, the forward voltage of the“diode” is higher than a silicon transistor. Significantly, by applyinga negative V_(GS) bias to the GaN device (as shown in FIG. 7 of theaccompanying drawings) this “diode” forward voltage can be furtherincreased to several volts.

Due to the lack of the parasitic MOSFET body diode behaviour, using GaNtransistors for phase isolation reduces the number of semiconductorswitches required for the complete solid state phase isolation relay(SSPIR) implementation from 4 to 2 in this embodiment, where it ismerely desired to limit the unwanted current 10 in case of a switchfailure. As such, the circuit of this embodiment is equivalent infunction to that of FIG. 1 or 2 of the accompanying drawings.

In normal operation each GaN transistor 62, 63 is turned on by apositive voltage Vgs between the gate and source terminals in a similarmanner to a MOSFET.

Under fault conditions such as a short circuit inverter MOSFET 51, thephase isolation GaN switches 62, 63 can be turned off to preventuncontrolled currents circulating in the motor phases. This can becarried out by controlling the gate-source voltage of each GaN switch62, 63 depending on the voltage across the switching element.

For a positive voltage across the GaN switch 62, 63, the gate-sourcevoltage may be maintained close to 0V. For a negative voltage across theGaN switch 62, 63, the applied gate-source voltage is negative. Thenegative gate-source voltage to be applied may be determined by thevoltage across the switch 62, 63, as discussed below with respect toFIG. 6.

FIG. 5 of the accompanying drawings shows a second embodiment of theinvention, which is equivalent in function to that of FIG. 3 of theaccompanying drawings. Features equivalent to that of the circuit ofFIG. 4 have been shown with corresponding reference numerals, raised by50.

In this embodiment, there are three phase isolation switches 111, 112,113 all employing GaN transistors. As these devices can block current ineither direction, the number of phase isolation semiconductor switchesin order to allow control of the non-failed phases (as in FIG. 3 above)reduces from 6 to 3. Both embodiments therefore have the benefit ofhaving lower power component count, cost and lower inserted losses.

A GaN phase isolation driver circuit 120 to achieve the behaviour aboveis shown in FIG. 6; this can be used to control the GaN transistors ofthe circuits of either FIG. 4 or FIG. 5. For the sake of descriptiononly, we describe its use in controlling switch 62 of FIG. 4, but theprinciple of operation would be the same for any of the phase isolationswitches 62, 63, 111, 112, 113 described above.

In normal operation switch SW1 is closed and the driver circuit 120applies a voltage between the gate and source of the GaN switch 62(V_(gs)) appropriate to fully turn on the GaN switch 62. R1 resistanceis relatively large in comparison to R2 and therefore it does notsignificantly affect the gate voltage. With the GaN switches 62, 63;111, 112, 113 closed in each phase current can be freely controlled inthe motor 57, 107 with the three phase inverter 51, 52, 53, 54, 55, 56;101, 102, 103, 104, 105, 106.

In case of a short circuit failure of one of the inverter MOSFETS 51,52, 53, 54, 55, 56; 101, 102, 103, 104, 105, 106 when it is desired toturn off the GaN switch 62, the voltage output by the driver circuit 120can be controlled to 0V and SW1 opened. Looking at the voltage acrossthe GaN switch 62 (that is, the voltage between the drain and the sourceV_(ds)), for positive V_(ds) conditions, V_(gs) is held close to 0V byR2 in series with the diode across SW1. For negative V_(ds) conditions,with the diode across SW1 being reverse biased, the gate voltage isdetermined by R1 and the drain voltage.

The invention claimed is:
 1. A control circuit for a multi-phase motor,the control circuit comprising: a plurality of inverter bridges, eachinverter bridge being arranged to provide an output voltage for a phaseof the motor; a plurality of outputs, each output being arranged to becoupled to one phase of the motor to provide the output voltage to thatphase of the motor; at least one isolation switch, each isolation switchbeing coupled between one of the inverter bridges and one of theoutputs, so as to selectively isolate the output from the inverterbridge, in which the at least one isolation switch is a Gallium Nitride(GaN) transistor comprising a gate, a drain, and a source; a controllercomprising a first output, and a second output coupled to the gate ofthe GaN transistor; a switch and a diode connected in parallel betweenthe first output and the source of the GaN transistor; and whereinduring a normal mode of operation of the controller, the switch isclosed and the diode is forwarded biased and the controller applies apositive voltage between the gate and the source of the GaN transistorso that the GaN transistor does not isolate its output from its inverterbridge when the switch is closed, and wherein during a phase isolationmode of operation of the controller, the switch is opened and the diodeis reverse biased and the controller applies one of a zero or a negativevoltage between the gate and the source of the GaN transistor so thatthe GaN transistor isolates its output from the inverter bridge when theswitch is opened.
 2. The control circuit of claim 1, in which each GaNtransistor provides the selective isolation of each isolation switch. 3.The control circuit of claim 1, in which each inverter bridge which isnot coupled to an isolation switch is coupled to an output.
 4. Thecontrol circuit of claim 1, in which the controller is arranged suchthat the negative voltage applied between the gate and the source ofeach GaN transistor may be dependent upon a voltage across the GaNtransistor.
 5. The control circuit of claim 1, wherein the controllerapplies the zero voltage between the gate and the source of the GaNtransistor when the GaN transistor has a positive drain to sourcevoltage.
 6. The control circuit of claim 1, wherein the controllerapplies the negative voltage between the gate and the source of the GaNtransistor when the GaN transistor has a negative drain to sourcevoltage.
 7. A motor apparatus comprising a motor having a plurality ofphases and a control circuit, wherein the control circuit comprises: aplurality of inverter bridges, each inverter bridge being arranged toprovide an output voltage for a phase of the motor; a plurality ofoutputs, each output being arranged to be coupled to one phase of themotor to provide the output voltage to that phase of the motor; at leastone isolation switch, each isolation switch being coupled between one ofthe inverter bridges and one of the outputs, so as to selectivelyisolate the output from the inverter bridge, in which the at least oneisolation switch is a Gallium Nitride (GaN) transistor comprising agate, a drain, and a source, and wherein each phase is coupled to one ofthe outputs of the control circuit; a controller comprising a firstoutput, and a second output coupled to the gate of the GaN transistor; aswitch and a diode connected in parallel between the first output andthe source of the GaN transistor; and wherein during a normal mode ofoperation of the controller, the switch is closed and the diode isforwarded biased and the controller applies a positive voltage betweenthe gate and the source of the GaN transistor so that the GaN transistordoes not isolate its output from its inverter bridge when the switch isclosed, and wherein during a phase isolation mode of operation of thecontroller, the switch is opened and the diode is reverse biased and thecontroller applies one of a zero or a negative voltage between the gateand the source of the GaN transistor so that the GaN transistor isolatesits output from the inverter bridge when the switch is opened.
 8. Themotor apparatus of claim 7, in which each GaN transistor provides theselective isolation of each isolation switch.
 9. The motor apparatus ofclaim 7, in which each inverter bridge which is not coupled to anisolation switch is coupled to an output.
 10. A method of controlling acontrol circuit for a multi-phase motor, the control circuit comprising:a plurality of inverter bridges, each inverter bridge being arranged toprovide an output voltage for a phase of the motor; a plurality ofoutputs, each output being arranged to be coupled to one phase of themotor to provide the output voltage to that phase of the motor; at leastone isolation switch, each isolation switch being coupled between one ofthe inverter bridges and one of the outputs, so as to selectivelyisolate the output from the inverter bridge, in which the each isolationswitch comprises a Gallium Nitride (GaN) transistor comprising a gate, adrain, and a source; a controller comprising a first output, and asecond output coupled to the gate of the GaN transistor; a switch and adiode connected in parallel between the first output and the source ofthe GaN transistor; and the method comprising, in a normal mode ofoperation of the controller: closing the switch to forward bias thediode; and applying a positive voltage between the gate and the sourceof the GaN transistor so that the GaN transistor does not isolate itsoutput from its inverter bridge when the switch is closed; and themethod further comprising, in a phase isolation mode of operation of thecontroller: opening the switch to reverse bias the diode; and one ofapplying a zero or a negative voltage between the gate and the source ofthe GaN transistor so that the GaN transistor isolates its output fromthe inverter bridge when the switch is open.
 11. The method of claim 10,in which the negative voltage applied between the gate and the source ofeach GaN transistor is dependent upon a voltage across the GaNtransistor.